Internal API for custom waiting loops

To be used by kotlinx-io

Author: elizarov

core/kotlinx-coroutines-core/src/EventLoop.kt

@@ -315,4 +315,27 @@ internal class BlockingEventLoop(
     override val thread: Thread
 ) : EventLoopImplBase()
 
-internal actual fun createEventLoop(): EventLoop = BlockingEventLoop(Thread.currentThread())
+internal actual fun createEventLoop(): EventLoop = BlockingEventLoop(Thread.currentThread())
+
+/**
+ * Processes next event in the current thread's event loop.
+ *
+ * The result of this function is to be interpreted like this:
+ * * `<= 0` -- there are potentially more events for immediate processing;
+ * * `> 0` -- a number of nanoseconds to wait for the next scheduled event;
+ * * [Long.MAX_VALUE] -- no more events, or was invoked from the wrong thread.
+ *
+ * Sample usage of this function:
+ *
+ * ```
+ * while (waitingCondition) {
+ *     val time = processNextEventInCurrentThread()
+ *     LockSupport.parkNanos(time)
+ * }
+ * ```
+ *
+ * @suppress **This an internal API and should not be used from general code.**
+ */
+@InternalCoroutinesApi
+public fun processNextEventInCurrentThread(): Long =
+    ThreadLocalEventLoop.currentOrNull()?.processNextEvent() ?: Long.MAX_VALUE